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Vehicle Transmission

By R&D
Published on December 29th, 2023

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The Transmission is the vehicle's gearbox. The transmission components are always mounted straight on the engine so that their attached belt and gear system can effectively convert the combustion power produced by the engine into physical momentum.

How the Vehicle's Engine Operates:

  • Fuel is ignited in the combustion chamber of your vehicle’s engine.
  • As fuel ignites, the expanding gas and heat from the miniature explosion push the pistons inside your engine’s cylinders.
  • As the pistons are pushed, they move up and down, and spin your engine’s crankshaft. 
  • The crankshaft then turns your car’s drive wheels. Through this mechanism, explosive energy is turned into mechanical energy.

A typical vehicle transmission is usually made up of five and six gear sets, and a series of gear trains that allows a driver to control how much power is delivered to the car without changing how fast the engine runs. 

A vehicle's transmission helps to ensure that your engine spins at the right rate without going too fast or too slow for your needs. Also, it ensures that your wheels get the correct amount of power. Without a transmission, any vehicle would be difficult to start, stop, and would be completely unreliable.

How do Transmissions Work?

A transmission changes gears depending on car speed and accelerator input so that the engine’s RPM are kept appropriately low. This provides two benefits:

  • Fuel consumption is decreased.
  • The turning gears do not overload your engine.

Additionally, a transmission allows you to harness your vehicle’s engine energy when necessary. When you’re driving slowly, you can stay in a lower gear and avoid damaging your engine. When you must move quicker, you can switch gears to a higher level. By doing so, the transmission allows you to benefit from more of your vehicle's power, rotating the wheels a lot quicker.

Regardless of whether your vehicle has a manual or automatic transmission, all car gearboxes work in essentially the same way. The only difference is whether the driver must manually push the clutch to disconnect the engine and transmission and move the car into a new gear level.

  • A gear lever selects and moves gears to connect with one another. The driver operates the gear lever using the clutch control/pedal (if manual). Otherwise, this is performed automatically.
  • When engaged, a clutch or gear lever maneuvers Collar plates (also called clutch plates) into place to connect to larger gears, which are themselves connected to your car’s differential.
  • As the gear lever moves, different gears may be connected at different times. This alters which sets of gears turn and the power ratio delivered from the engine to wheels.

Ever wondered why your car’s engine might make a horrible sound if you engage the clutch incorrectly? It’s not because the gear teeth are mismatched, as most would think. In modern transmissions, gear teeth are positioned to be fully engaged at all times, even gears that aren’t technically in play. This is called Freewheeling. Instead, that horrible grinding sound occurs when the collar plate’s Dog Teeth or Connecting Notches don’t match up with the right holes in the side of a transmission gear.

Manual Transmission:

With a manual transmission, the clutch must be engaged such that the collar plate disconnects fully from transmission gears. This is why when engaging a manual transmission and making a change from one gear to another, a car might seem to lag or jump.

The clutch must then be maneuvered so that the collar plate lines up perfectly with the transmission gears you want to select. If done correctly, your car will immediately begin to drive at your selected gear.

Vehicle Transmission image 1.png__PID:6a1a59eb-5cc1-4600-b3c0-477f6a1110ce

Five-Speed Manual gearbox and Power Flows

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RWD Manual Gearbox

Automatic Transmission:

An automatic transmission also features an automatic clutch. Instead of relying on human control, built-in sensors, processors, and actuators operate the clutch at the perfect moment based on your current speed, accelerator pressure, and other factors. This allows drivers to focus on other tasks on the road.

You can actually hear an automatic transmission operate as you drive a vehicle equipped with one. As you push down on the accelerator, your engine will become louder, and its built-in torque converter will sense the change and automatically shift to a higher gear. 

In contrast, a car with a manual transmission will begin to slow and struggle as it is pushed to higher speeds until you switch gears.

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Automatic Transmission

Torque Converter Operation

The Torque Converter is a device automatic transmissions use. It delivers power from the engine to the gearbox like a basic fluid flywheel while increasing the torque when the car begins to move. The torque converter resembles a large doughnut sliced in half. One half, called the Pump Impeller, is bolted to the drive plate or flywheel. The other half, called the Turbine, is connected to the gearbox input shaft. Each half is lined with Vanes or Blades. The pump and the turbine face each other in a case filled with oil. A bladed wheel called a Stator is fitted between them.

The engine causes the pump to rotate and throw oil against the vanes of the turbine. The force of the oil makes the turbine rotate and send power to the transmission. After striking the turbine vanes, the oil passes through the stator and returns to the pump. When the pump reaches a specific rate of rotation, a reaction between the oil and the stator increases the torque. In a fluid flywheel, oil returning to the impeller tends to slow it down. In a torque converter, the stator or reactor diverts the oil towards the center of the impeller for extra thrust.

When the engine is running slowly, the oil may not have enough force to rotate the turbine. But when the driver presses the accelerator pedal, the engine runs faster and so does the impeller. The action of the impeller increases the force of the oil. This force gradually becomes strong enough to rotate the turbine and move the vehicle. A torque converter can double the applied torque when moving off from rest. As engine speed increases, the torque multiplication tapers off until at cruising speed when there is no increase in torque. The reactor or stator then freewheels on its one-way clutch at the same speed as the turbine.

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Constantly Variable Transmission (CVT)

This kind of automatic transmission uses two pairs of cone-shaped pulleys connected by a metal belt. The key to this system is the high friction drive belt. The belt is made from high-performance steel and transmits drive by thrust rather than tension. The ratio of the rotations, often called the gear ratio, is determined by how far the belt rides from the centers of the pulleys. The transmission can produce an unlimited number of ratios. As the car changes speed, the ratio is continuously adjusted. Cars with this system are said to use fuel more efficiently than cars with set gear ratios. Within the gearbox hydraulic control is used to move the pulleys and hence change the drive ratio. An epicyclic gear set is used to provide a reverse gear as well as a fixed ratio.

Note: A CVT transmission can produce an unlimited number of ratios

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Constantly Variable Transmission

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Ravigneaux Gear Set

Epicyclic Gearbox Operation

Epicyclic gears are a special set of gears that are part of most automatic gearboxes. They consist of three elements:

  • A sun gear is located in the center.
  • The carrier that holds two, three or four planet gears, which mesh with the sun gear and revolve around it.
  • An internal gear or annulus (a ring with internal teeth), which surrounds the planet gears and meshes with them.

Any part of a set of planetary gears can be held stationary or locked to one of the others. This will produce different gear ratios. Most automatic gearboxes have two sets of planetary gears that are arranged in line. This provides the necessary number of gear ratios. The appropriate elements in the gear train are held stationary by a system of hydraulically operated brake bands and clutches. These are worked by a series of hydraulically operated valves in the lower part of the gearbox. Oil pressure to operate the clutches and brake bands is supplied by a pump.

Unless the driver moves the gear selector to operate the valves, automatic gear changes are made depending on just two factors:

  • Throttle opening – a cable is connected from the throttle to the gearbox
  • Road speed – when the vehicle reaches a set speed, a governor allows pump pressure to take over from the throttle. The cable from the throttle also allows a facility known as Kick Down. This allows the driver to change down a gear, such as for overtaking, by pressing the throttle all the way down.

Electronic Control of Transmission

The main aim of electronically controlled automatic transmission (ECAT) is to improve on conventional automatic transmission in the following ways:

  • Smoother and quieter gear changes
  • Improved performance
  • Reduced fuel consumption
  • Reduction of characteristic changes over system life
  • Increased reliability

The important points to remember are that gear changes and lock-up of the torque converter are controlled by hydraulic pressure. In an ECAT system, electrically controlled solenoid valves can influence this hydraulic pressure. Most ECAT systems now have a transmission ECU that is in communication with the engine control ECU.

With an ECAT system, the actual point of gear-shift is determined from pre-programmed memory within the ECU. Data from other sensors is also taken into consideration. Actual gearshifts are initiated by changes in hydraulic pressure, which is controlled by solenoid valves.

The two main control functions of this system are hydraulic pressure and engine torque. A temporary reduction in engine torque during gear shifting allows smooth operation. This is because the peak of gearbox output torque which causes the characteristic surge during gear changes on conventional automatics is suppressed. Because of these control functions smooth gearshifts are possible and, due to the learning ability of some ECUs, the characteristics remain throughout the life of the system.

The ability to lock up the torque converter has been used for some time even on vehicles with more conventional automatic transmission. This gives better fuel economy, quietness and improved drivability. Lock-up is carried out using a hydraulic valve, which can be operated gradually to produce a smooth transition. The timing of lock-up is determined from ECU memory in terms of the vehicle speed and acceleration.

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ECAT  Block Diagram

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Electrohydraulic Valve Block

Direct Shift Gearbox

The direct shift gearbox (DSG) is an interesting development as it could be described as a manual gearbox that can change gear automatically. It can be operated by Paddles behind the steering wheel, a lever in the center console or in a fully automatic mode. The gear train and synchronizing components are similar to a normal manual change gearbox. The direct shift gearbox is made of two transmission units that are independent of each other. Each transmission unit is constructed in the same way as a manual gearbox and is connected by a multiplate clutch. They're regulated, opened and closed by a mechatronics system. On the system outlined in this section:

  • 1st, 3rd, 5th and reverse gears are selected via multiplate clutch 1.
  • 2nd, 4th and 6th gears are selected via multiplate clutch 2.

Using the 7111 Tablet to Look at Transmission Live Data

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